# Tool router

**Source:** https://promtable.com/glossary/tool-router

> A tool router is a layer in an agent that decides which tool to call (or which sub-agent to delegate to) for a given step — distinct from a model router which picks the underlying LLM.

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A tool router is a layer in an agent that decides which tool to call (or which sub-agent to delegate to) for a given step — distinct from a model router which picks the underlying LLM.

As agent tool counts grow past ~20, frontier models start mis-routing at higher rates. A tool router solves this by classifying intent first (using a small cheap model) and exposing only the relevant tools to the executor for that step. The pattern composes with model routing: tool router picks the tool set, model router picks the model that handles it best. Frameworks like LangGraph, OpenAI Swarm, and Claude Agent SDK ship variants. Best practice in 2026 is to design agents as small specialised sub-agents rather than monolithic ones — natural tool routing falls out of architecture.

## When to use

- Agents with more than ~20 tools.
- Multi-agent architectures.

## Common mistakes

- Tool router that runs on every step — overhead can dominate.
- Skipping evals on the routing layer — routing accuracy is its own quality dimension.

## Related terms

- [agent](https://promtable.com/glossary/agent)
- [model-router](https://promtable.com/glossary/model-router)
- [function-calling](https://promtable.com/glossary/function-calling)
- [tool-use](https://promtable.com/glossary/tool-use)

*Last updated: 2026-06-01*
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Original page: https://promtable.com/glossary/tool-router
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